This invention relates to phased array antennas and particularly to apparatus for preventing the degradation of antenna beam pointing accuracy in the event of a phase shifter malfunction.
One of the characteristics of most phased array antenna systems, because of the large number of antenna elements which are usually provided, is the feature that a failure of signals supplied to any particular element does not result in an excessive degradation of antenna performance. The system therefore "fails soft" or slowly degrades as elements fail. In microwave landing systems, however, the antenna beam pointing accuracy is especially critical, so that aircraft angular location can be determined with precision. U.S. Pat. No. 4,041,501 to Frazita, et al. discloses a phased array antenna system which is useful for microwave landing systems, and which contains a reduced number of active phase shifters as compared to prior art phased array antennas. In antenna systems of this type, wherein accurate beam pointing is required, and wherein a single phase shifter provides signals to several antenna elements, a malfunction of one of the phase shifters can produce substantial degradation of antenna performance, particularly with respect to antenna beam pointing accuracy. According to prior art techniques, it is possible to detect the malfunction of a phase shifter by the use of built-in test equipment (BITE) at the phase shifter. BITE enables rapid and automatic detection of a phase shifter malfunction and enables the system operator to arrange for replacement of the failed component. While BITE ensures that a failure will be quickly detected, it is still necessary to discontinue use of the antenna system until the failed component is replaced if accurate system beam pointing is to be maintained. Such unscheduled system shut downs can be costly and hazardous, particularly at busy air terminals.
It is an object of the present invention to provide an apparatus which prevents a phase shifter malfunction from affecting the beam pointing accuracy of a phased array antenna, and therefore enables the antenna system to remain in operation until the malfunctioning phase shifter can be conveniently replaced.
It is a further object of the invention to provide such an apparatus which easily interfaces with existing phase shifter designs, and can be implemented at relatively small incremental cost for the antenna system.